1. Field of the Invention
The present invention relates to a video data storage and playback system in which video data stored at a storage device are transmitted to and playbacked at a playback device. Note that, in the following description, a term "overtaking" is used to indicate a situation in which the video data reading operation catches up with (but not passing) the video data writing operation.
2. Description of the Background Art
FIG. 5 shows a schematic configuration of a conventional video data storage and playback system, which generally comprises a storage device 1 and a playback device 2.
The storage device 1 further comprises a writing control unit 3, a memory unit 4, a writing position information register unit 5, and a reading control unit 6. This storage device 1 can be provided in a form of a video server in which digital video data under the writing processing can also be read out, for example, in which case the writing control unit 3 is formed by a network interface board and a control program provided in a computer.
Also, the memory unit 4 can be formed by an SCSI (Small Computer System Interface) board and a hard disk device provided in a computer, or by an IDE (Integrated Drive Electronics) controller and an optical disk device provided in a computer. The writing position information register unit 5 is formed on a main memory in a computer, while the reading control unit 6 is formed by a network interface board and a control program provided in a computer. The writing control unit 3 and the reading control unit 6 often shares the common network interface board.
The playback device 2 further comprises playback control buttons 7, a playback control unit 8, a decoding unit 9, a display unit 10 and a speaker 11.
This playback device 2 can be provided in a form of a video-on-demand terminal, for example, in which case the playback control buttons 7 are provided by a GUI (Graphical User Interface) displayed on the display unit 10.
Also, the playback control unit 8 is formed by a network interface board and a control program provided in a PC, while the decoding unit 9 can be formed by a network interface board and an MPEG (Moving Picture Experts Group) hardware decoder provided in an PC, or by an MPEG software decoder and the like. The playback control unit 8 and the decoding unit 9 often share the common network interface board.
Note that, in FIG. 5, each connection line with arrowhead indicates a route of video data (which will be referred to as a data line hereinbelow), where the direction of arrowhead indicates its transmission direction. The video data are transmitted through networks. Also, in FIG. 5, each connection line without arrowhead indicates a route of control information (which will be referred to as a control line hereinbelow), which is transmitted bidirectionally. In the case where the common network interface board is shared by the writing control unit 3 and the reading control unit 6, or by the playback control unit 8 and the decoding unit 9, the data lines and the control lines are often distinguished logically while using the same physical layer.
Now, a control method in the conventional video data storage and playback system of FIG. 5 in the case where the playback chases after the video data under the writing processing will be described.
FIG. 6 shows a flow chart for the processing procedure by the storage device 1 of FIG. 5, while FIG. 7 shows a flow chart for the processing procedure by the playback device 2 of FIG. 5.
First, a user clicks a fast forward button (or a playback button) among the playback control buttons 7 displayed on the display unit 10 of the playback device 2 using a mouse.
Then, when the chasing playback control processing of FIG. 7 begins upon detecting this operation (step 210), the playback control unit 8 of the playback device 2 commands the reading processing to the reading control unit 6 by specifying a reading mode selected by the user (step 211), and starts the video data reception and the playback processing in the specified mode (step S212).
Thereafter, the playback control unit 8 checks whether an overtaking notice from the reading control unit 6 of the storage device 1 is received or not (step S213). Here, the overtaking occurs when the video data reading is carried out faster than the video data writing at the storage device 1. This overtaking often occurs when video data are to be playbacked in the fast forward mode at the playback device 2, or when a rate of decoding at the decoding unit 9 of the playback device 2 is higher than a rate of receiving video data from the external at the storage device 1 even in the case of playbacking video data in the normal speed at the playback device 2. Besides these, the overtaking also occurs when the video data writing is temporarily stopped for some reason such as a temporal interruption of the video data that have been inputted into the writing control unit 3.
At the storage device 1, the writing control unit 3 receives video data transmitted from the external and writes them into the memory unit 4 sequentially, while registering a position information of each writing completed video data among a series of video data into the writing position information register unit 5.
Then, in parallel to the above operation, when the chasing playback control processing of FIG. 6 begins upon receiving a reading command from the playback device 2 (step 200), the reading control unit 6 starts the processing for reading from the memory unit 4 in the specified mode and transmitting the read out video data to the decoding unit 9 of the playback device 2 (step 201).
Thereafter, the reading control unit 6 reads out the position information of each writing completed video data from the writing position information register unit 5, and checks whether the video data to be read out exists in the memory unit 4 or not, that is, whether the overtaking has occurred or not (step 202). If no overtaking has occurred the video data reading and transmission processing is continued.
At the playback device 2, the decoding unit 9 decodes the received video data, and transmits video image data to the display unit 10 and audio data to the speaker 11. Then, the display unit 10 displays the received video image data. In the case of the playback in the fast forward mode, it is customary to playback no audio data.
On the other hand, when the overtaking occurs, as shown in FIG. 6, the reading control unit 6 of the storage device 1 stops the reading processing (step 203), and notifies the occurrence of the overtaking to the playback control unit 8 of the playback device (step 204). Then, this chasing playback control processing of FIG. 6 is terminated (step 220).
Also, when the overtaking notice is received at the step 213, the playback control unit 8 of the playback device 2 controls the decoding unit 9 such that the decoding unit 9 playbacks the video data remaining in buffer memories within the reading control unit 6 and the decoding unit 9 and then pauses (step 214), and sets only a pause button among the playback control buttons 7 in an operable state (step 230). Then, this chasing playback control processing of FIG. 7 is terminated (step 240).
Also, when the overtaking occurs, in order to continue the playback of the video data automatically without terminating the chasing playback control processing, the following operation sequence may be considered.
Namely, after the overtaking occurs, the playback control unit 8 receives a video data reading interruption notice attached with indicating the occurrence of the overtaking and a playback position information indicating a playback position at which the overtaking occurred, through the control line from the reading control unit 6, and then controls the decoding unit 9 such that the decoding unit 9 playbacks all the video data remaining in buffer memories within the reading control unit 6 and the decoding unit 9 so as to vacate the both buffer memories. Then, the playback control unit 8 transmits a request for reading in the ordinary playback mode starting from a position next to the playback position at which the overtaking occurred that has been notified earlier, through the control line to the reading control unit 6. Then, the playback control unit 8 receives a reading request acknowledgement through the control line from the reading control unit 6. When this received reading request acknowledgement indicates OK, the playback control unit 8 is set in a data waiting state, and the decoding unit 9 resumes receiving of the video data through the data line from the reading control unit 6. When the received reading request acknowledgement indicates NG, the playback control unit 8 waits for awhile and then again transmits a request for reading in the ordinary playback mode from a position next to the playback position at which the overtaking occurred that has been notified earlier, through the control line to the reading control unit 6.
As described above, in the conventional video data storage and playback system of FIG. 5, when the overtaking occurs, the storage device transmits the overtaking notice to the playback device 2 such that the playback device 2 makes a transition to the pause state upon receiving this overtaking notice from the storage device 1.
For this reason, there has been a problem that, in order to playback the video data after the overtaking occurs, the user of the playback device 2 is forced to carry out tedious operations of waiting for awhile and then trying to click the playback button with the mouse.
On the other hand, in the above described operation sequence for continuing the playback of the video data automatically without terminating the chasing playback control processing when the overtaking occurs, the video data on the buffer memories are eliminated once by playbacking all the video data remaining in buffer memories within the reading control unit 6 and the decoding unit 9 so as to vacate the both buffer memories, rather than leaving the video data on the buffer memories and treating them continuously with the video data of the resumed playback after the overtaking. This is because it has been difficult for the playback control unit 8 to comprehend the exact timing of the video data from which the resumed playback after the overtaking in the ordinary mode should be carried out, and realize the continuous playback by accurately aligning the display of the video data on the buffer memory in the decoding unit 9 by the display unit 10 and the playback mode dependent state of the playback control buttons 7. Note that this is the problem caused by the fact that the logically distinguished data lines and control lines are not necessarily synchronized with each other because of the delay through the communication channels or the particulars of the communication control scheme. For example, when the information indicating the occurrence of the overtaking arrives through the control line with a considerable delay compared with the data transmitted through the data line, the continuous playback becomes impossible because the next reading request cannot be transmitted at appropriate timing. When there is a considerable delay until it becomes possible to resume the playback after the overtaking, there can be a state of having no video data to be displayed and in such a state the video display can be interrupted and the entirely black display may be presented depending on types of the decoder 9 and the display unit 10, so as to damage the video display continuity.